Flexural Strength Comparison of Silorane- and Methacrylate-Based Composites with Pre-impregnated Glass Fiber.

STATEMENT OF THE PROBLEM
Sufficient adhesion between silorane/methacrylate-based composites and methacrylate impregnated glass fiber increases the benefits of fibers and enhances the mechanical and clinical performance of both composites.


PURPOSE
The aim of this study was to evaluate the compatibility of silorane and methacrylate-based composites with pre-impregnated glass fiber by using flexural strength (FS) test.


MATERIALS AND METHOD
A total of 60 bar specimens were prepared in a split mold (25×2×2 mm) in 6 groups (n=10). In groups 1 and 4 (control), silorane-based (Filtek P90) and nanohybrid (Filtek Z350) composites were placed into the mold and photopolymerized with a high-intensity curing unit. In groups 2 and 5, pre-impregnated glass fiber was first placed into the mold and after two minutes of curing, the mold was filled with respective composites. Prior to filling the mold in groups 3 and 6, an intermediate adhesive layer was applied to the glass fiber. The specimens were stored in distilled water for 24 hours and then their flexural strength was measured by 3 point bending test, using universal testing machine at the crosshead speed of 1 mm/min. Two-way ANOVA and post-hoc test were used for analyzing the data (p< 0.05).


RESULTS
A significant difference was observed between the groups (p< 0.05). The highest FS was registered for combination of Z350 composite, impregnated glass fiber, and application of intermediate adhesive layer .The lowest FS was obtained in Filtek P90 alone. Cohesive failure in composite was the predominant failure in all groups, except group 5 in which adhesive failure between the composite and fiber was exclusively observed.


CONCLUSION
Significant improvement in FS was achieved for both composites with glass fiber. Additional application of intermediate adhesive layer before composite build up seems to increase FS. Nanohybrid composite showed higher FS than silorane-based composite.


Introduction
Resin-based composites have been the material of choice for most restorations in dental practice during the last decade. [1] Since the development of composite resin, several modifications have been made to reduce their limitations like mechanical deficiencies, polymerization shrinkage, and degradation in oral environment. [2][3] Impregnated fiber-reinforced composite (FRC) resin can be used as filler into the resin composite matrix, or separately in conjunction with the resin compo-site. Both forms are acceptable approaches to enhance the mechanical properties of composite resin to function well in oral cavity. [1,4] Many researches supported the satisfactory handling properties and adequate clinical performance of FRC. [5-6] They could be used in periodontal splint, avulsed teeth splint, endodontic post and cores, orthodontic retainers reinforcement and recently, in fixed partial dentures. [7][8] Fiber impregnation with adhesive resin may be an important factor in developing the adhesive strength between the fiber and composite materials by transferring the loading forces from the resin matrix to the fiber. [9] The two resin systems which are used for impregnation of fiber employ photopolymerizable dimethacrylate monomer and a combination of dimethacrylate monomer resin along with liner polymer which form the semi-interpenetrating polymer network. [10][11] [14] The insertion of fibers also significantly reduced the microleakage in class II composite restorations. [15][16][17] Recently, ring-opening silorane-based composites have been increasingly used to overcome the shortcoming of methacrylate resin composites and improve their clinical performance. [18] It was demonstrated that the pre-impregnated glass fiber along with silorane-based composites can create better result in cuspal deflection and fracture resistance of endodontically treated premolar. [19] Polacek et al. showed that application of an intermediate layer of adhesive resin on the pre-impregnated glass fiber before composite build-up yielded higher shear bond strength (SBS). [20] Many researchers postulated that flexural strength is a good indicator for clinical performance of FRC, especially in FRC fixed partial denture. [12][13][14]21] To the best of the authors' knowledge, no infor- To determine the flexural strength, a stainless steel mold (2×2×25 mm) was fabricated according to ISO 4049 and used to obtain 60 rectangular composite bars in 6 groups (n=10) as follows.
In groups 1 and 4 (control groups), Filtek Z350 and Filtek P90 composites were placed into the mold in one increment and were completely condensed. The surface of the mold was covered with a Mylar strip, light polymerized for 40 seconds with VIP Junior curing unit (Bisco; Schaumberg, USA) at a 600 mW/cm 2 light intensity, and cured in three separate overlapping techniques. After 10 minutes, the composite bars were removed from the mold and the above-mentioned curing procedure was repeated on the other side surfaces.
In groups 2 and 5, a 25-mm pre-impregnated glass fiber was cut and put at the bottom of the mold. Several

Results
The mean and standard deviation (SD) of flexural strength for each group are summarized in Table 1.  Filtek Z350 composite had significantly higher flexural strength than Filtek P90 composite (p= 0.002). The failure mode of each fiber-composite specimen is represented in Table 2. Most failure modes in were found to be mode A, except in group 5 which predominantly had failure mode B.

Discussion
Currently, the interest for using FRC is rapidly increasing and this material seems to be accepted by clinicians for many clinical applications in dentistry [9,22] In the current study, a pre-impregnated glass fiber was em-  Analysis of the failure mode in our study showed that the failure mode of fiber-composite specimen in group 6 shifted toward the cohesive failure in silorane composite when compared with predominantly adhesive failures in group 5. Although both groups had significantly higher flexural strength than group 4, it confirms the previous results that a good adhesion between silorane composite and glass fiber was created in group 6.
In our study, nano hybrid composite (without fiber) exhibited significantly higher flexural strength than silorane composite.
Several studies demonstrated a direct relation between filler volume content, size and shape on mechanical properties. [33] Flexural strength generally is reduced by increasing the filler particle size. [34] As mentioned in Table 3, the filler volume and size of Filtek